Methods for forming security articles having diffractive surfaces and color shifting backgrounds

ABSTRACT

A security article includes a light transmissive substrate having a first surface and an opposing second surface, with the first surface having an embossed region with an optical diffraction pattern or a holographic image pattern. A color shifting optical coating is formed on the substrate such as on the opposing second surface, with the optical coating providing an observable color shift as the angle of incident light or viewing angle changes. The security article can be used in a variety of applications and products to provide for enhanced security measures such as anticounterfeiting.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 09/351,102filed on Jul. 8, 1999, now U.S. Pat. No. 6,761,959, entitled“Diffractive Surfaces With Color Shifting Backgrounds,” whichapplication is incorporated herein in its entirety, and to whichpriority is claimed.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention is related generally to thin film optical coatingsfor use in producing security articles. More specifically, the presentinvention is related to the production of diffractive surfaces such asholograms or gratings having color shifting or optically variablebackgrounds which can be used as security articles in a variety ofapplications.

2. The Relevant Technology

Color shifting pigments and colorants have been used in numerousapplications, ranging from automobile paints to anti-counterfeiting inksfor security documents and currency. Such pigments and colorants exhibitthe property of changing color upon variation of the angle of incidentlight, or as the viewing angle of the observer is shifted. The primarymethod used to achieve such color shifting colorants is to dispersesmall flakes, which are typically composed of multiple layers of thinfilms having particular optical characteristics, throughout a mediumsuch as paint or ink that may then be subsequently applied to thesurface of an object.

Diffraction patterns and embossments, and the related field ofholographs, have begun to find wide-ranging practical applications dueto their aesthetic and utilitarian visual effects. One very desirabledecorative effect is the iridescent visual effect created by adiffraction grating. This striking visual effect occurs when ambientlight is diffracted into its color components by reflection from thediffraction grating. In general, diffraction gratings are essentiallyrepetitive structures made of lines or grooves in a material to form apeak and trough structure. Desired optical effects within the visiblespectrum occur when diffraction gratings have regularly spaced groovesin the range of hundreds to thousands of lines per millimeter on areflective surface.

Diffraction grating technology has been employed in the formation oftwo-dimensional holographic patterns which create the illusion of athree-dimensional image to an observer. Furthermore, the use ofholographic images on various objects to discourage counterfeiting hasfound widespread application.

There currently exist several applications for surfaces embossed withholographic patterns which range from decorative items, such as giftwrap, to security documents, such as bank notes and credit cards.Two-dimensional holograms typically utilize diffraction patterns whichhave been formed on a plastic surface. In some cases, a holographicimage which has been embossed on such a surface can be visible withoutfurther processing; however, it is generally necessary, in order toachieve maximum optical effects, to place a reflective layer, typicallya thin metal layer such as aluminum, onto the embossed surface. Thereflective layer substantially increases the visibility of thediffraction pattern embossment.

Unfortunately, there exists a substantial incentive for counterfeitersto reproduce the holograms which are frequently used in credit cards,bank notes, and the like. One of the methods used to reproduce hologramsis to scan a laser beam across the embossed surface and optically recordthe reflected beam on a layer of a material such as a photopolymerizablepolymer. The original pattern can subsequently be reproduced as acounterfeit. Another method is to remove the protective coveringmaterial from the embossed metal surface by ion etching, and then whenthe embossed metal surface is exposed, a layer of metal such as silver(or any other easily releasable layer) can be deposited. This isfollowed by deposition of a layer of nickel, which is subsequentlyreleased to form a counterfeiting embossing shim.

Due to the level of sophistication of counterfeiting methods, it hasbecome necessary to develop more advanced security measures. Oneapproach, as disclosed in U.S. Pat. Nos. 5,629,068 and 5,549,774 toMiekka et al., is the application of inks, such as metallic flake inks,metallic effect inks, or inks with pigments formed of optical stacks,upon the embossed surface in lieu of a thin metal layer. In anotherapproach, disclosed in U.S. Pat. Nos. 5,624,076 and 5,672,410 also toMiekka et al., embossed metal particles or optical stack flakes are usedto produce a holographic image pattern.

Another problem with the holographic images as described above is thatthey require direct specular illumination in order to be visualized.This means that for best viewing results, the illuminating light must beincident at the same angle as the viewing angle. Therefore, diffuselight sources, such as ordinary room lights or viewing by an overcastsky, when used to illuminate the holographic image, do not reveal muchof the visual information contained in the hologram, and what istypically seen is only a silver colored reflection from the embossedsurface.

It would therefore be of substantial advantage to develop improvedsecurity products to provide enhanced viewing qualities in ordinary roomlight and which are usable in various security applications to makecounterfeiting more difficult.

SUMMARY AND OBJECTS OF THE INVENTION

It is a primary object of the invention to provide a security articlehave color shifting properties which increases the difficulty ofcounterfeiting in a variety of applications.

Another object of the invention to provide a security article with adistinctive pattern that is readily observable over a wide range ofviewing angles in diffuse lighting conditions.

Another object of the invention is to provide a security article with aholographic pattern that has enhanced visibility and contrast to providefor viewing under diffuse lighting conditions without the need fordirect specular light.

Another object of the invention to provide a security article that canbe manufactured at low cost compared to prior security products.

To achieve the forgoing objects and in accordance with the invention asembodied and broadly described herein, a security article is providedwhich includes a light transmissive substrate having a first surface andan opposing second surface, with the first surface having an opticalinterference pattern such as a diffraction grating pattern or aholographic image pattern. A color shifting optical coating is formed onthe substrate, with the optical coating providing an observable colorshift as the angle of incident light or viewing angle changes. In oneembodiment, the color shifting optical coating is formed on the secondsurface of the substrate opposite from the optical interference pattern,and includes an absorber layer formed adjacent to the substrate, adielectric layer formed on the absorber layer, and a reflector layerformed on the dielectric layer. Alternatively, this multilayer opticalcoating can be formed on the same side of the substrate as theinterference pattern.

In another embodiment, the color shifting optical coating is applied tothe substrate in the form of a paint or ink which includes a polymericmedium and a plurality of color shifting multilayer optical interferenceflakes dispersed in the polymeric medium. In other embodiments, thecolor shifting optical coating is coextruded with a light transmissiveembossed substrate to form adjacent layers or is dispersed in the formof interference flakes in the substrate material prior to forming thesubstrate.

The security article of the invention can be used in a variety ofapplications to provide for enhanced security measures such asanticounterfeiting. The security article can be utilized in the form ofa label, a tag, a ribbon, a security thread, and the like, forapplication in a variety of objects such as security documents, monetarycurrency, credit cards, merchandise, etc.

These and other aspects and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully understand the manner in which the above-recitedand other advantages and objects of the invention are obtained, a moreparticular description of the invention will be rendered by reference tospecific embodiments thereof which are illustrated in the appendeddrawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be considered aslimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of accompanyingdrawings in which:

FIG. 1A is a schematic depiction of a security article having a colorshifting optical coating according to one embodiment of the presentinvention;

FIG. 1B is a schematic depiction of a security article having a colorshifting optical coating according to an alternative embodiment of thepresent invention;

FIG. 2A is a schematic depiction of a security article having a colorshifting optical coating according to another embodiment of the presentinvention;

FIG. 2B is a schematic depiction of a security article having a colorshifting optical coating according to an alternative embodiment of thepresent invention;

FIG. 3 is a schematic depiction of a security article according to yetanother embodiment of the present invention;

FIG. 4 is a schematic depiction of a security article according to afurther embodiment of the present invention;

FIG. 5 is a schematic depiction of the security article of FIG. 1A witha release layer formed thereon;

FIG. 6 is a schematic depiction of the security article of FIG. 1Aattached to a carrier substrate;

FIG. 7 is a schematic depiction of the security article of FIG. 1B witha release layer formed thereon; and

FIG. 8 is a schematic depiction of the security article of FIG. 1Battached to a carrier substrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to security articles havingdiffractive surfaces with color shifting backgrounds that produceenhanced visual effects. The configuration of the security articles issuch that a combination of either holographic or diffraction gratingpatterns with color shifting films or layers decreases the possibilityof counterfeiting. Furthermore, the article of the invention allows auser to more easily view the image or diffraction effect in diffuselight without the need for direct specular light.

Generally, the configuration of the security articles of the presentinvention is such that the combination of a light transmissivesubstrate, having an interference pattern on the surface thereof, withcolor shifting optical coatings provides security features that makeforgery or counterfeiting of an object difficult.

Referring to the drawings, wherein like structures are provided withlike reference designations, FIG. 1A depicts a security article 10according to one embodiment of the present invention. The securityarticle 10 includes a light transmissive substrate 14 formed with anoptical interference pattern 15 on an outer first surface thereof. Acolor shifting optical coating 16 is formed on an opposing secondsurface of substrate 14 and is discussed in further detail below. Thecombination of substrate 14 and color shifting optical coating 16forming security article 10 provide a security feature that reduces thepossibility of duplication, forgery and/or counterfeiting of an objecthaving security article 10 thereon.

The optical interference pattern 15 formed on the outer surface of lighttransmissive substrate 14 can take various conventional forms includingdiffraction patterns such as diffraction gratings, refraction patterns,holographic patterns such as two-dimensional and three-dimensionalholographic images, corner cube reflectors, or other like interferencepatterns. The particular methods and structures that form opticalinterference pattern 15 are known by those skilled in the art. Forexample, embossing the light transmissive substrate to form aninterference pattern thereon can be done by well known methods, such asembossing the surface of a plastic film by pressing it in contact with aheated nickel embossing shim at high pressure. Other methods includephotolithography, molding of the plastic film against a patternedsurface, and the like.

Generally, moldable materials are used to form light transmissivesubstrate 14 and include, for example, plastics such as polyethyleneterephthalate (PET), especially PET type G, polycarbonate, acrylics suchas polyacrylates including polymethyl methacrylate (PMMA),polyacrylonitrile, polyvinyl chloride, polystyrene, polypropylene,polynaphthalene terephthalate (PNT), mixtures or copolymers thereof, andthe like. It is preferred that light transmissive substrate 14 besubstantially composed of a transparent material such as polycarbonate.The substrate 14 is formed to have a suitable thickness of about 5 μm toabout 100 μm, and preferably a thickness of about 12 μm to about 25 μm.In addition, substrate 14 can be made of one layer or multiple layers ofsubstrate materials.

In one embodiment, substrate 14 can be produced from a thermoplasticfilm that has been embossed by heat softening the surface of the filmand then passing the film through embossing rollers which impart thediffraction grating or holographic image onto the softened surface. Inthis way, sheets of effectively unlimited length can be formed with thediffraction grating or holographic image thereon.

As shown in FIG. 1A, the color shifting optical coating 16 is amultilayer optical interference film that includes an absorber layer 18,a dielectric layer 20, and a reflector layer 22. The absorber layer 18is deposited on light transmissive substrate 14 by a conventionaldeposition process such as physical vapor deposition (PVD), sputtering,or the like. The absorber layer 18 is formed to have a suitablethickness of about 30-150 Angstroms, and preferably a thickness of about50-100 Angstroms. The absorber layer 18 can be composed of a semi-opaquematerial such as a grey metal, including metals such as chromium,nickel, titanium, vanadium, cobalt, and palladium, as well as othermetals such as iron, tungsten, molybdenum, niobium, aluminum, and thelike. Various combinations and alloys of the above metals may also beutilized, such as Inconel (Ni—Cr—Fe). Other absorber materials may alsobe employed in absorber layer 18 including metal compounds such as metalfluorides, metal oxides, metal sulfides, metal nitrides, metal carbides,metal phosphides, metal selenides, metal silicides, and combinationsthereof, as well as carbon, germanium, cermet, ferric oxide, metalsmixed in a dielectric matrix, and the like.

The dielectric layer 20 is formed on absorber layer 18 by a conventionaldeposition process such as PVD, reactive DC sputtering, RF sputtering,or the like. The dielectric layer 20 is formed to have an effectiveoptical thickness for imparting color shifting properties to securityarticle 10. The optical thickness is a well known optical parameterdefined as the product ηd, where η is the refractive index of the layerand d is the physical thickness of the layer. Typically, the opticalthickness of a layer is expressed in terms of a quarter wave opticalthickness (QWOT) that is equal to 4 ηd/λ, where λ is the wavelength atwhich a QWOT condition occurs. The optical thickness of dielectric layer20 can range from about 2 QWOT at a design wavelength of about 400 nm toabout 9 QWOT at a design wavelength of about 700 nm, and preferably 2-6QWOT at 400-700 nm, depending upon the color shift desired. Suitablematerials for dielectric layer 20 include those having a “high” index ofrefraction, defined herein as greater than about 1.65, as well as thosehave a “low” index of refraction, which is defined herein as about 1.65or less.

Examples of suitable high refractive index materials for dielectriclayer 20 include zinc sulfide (ZnS), zinc oxide (ZnO), zirconium oxide(ZrO₂), titanium dioxide (TiO₂), carbon (C), indium oxide (In₂O₃),indium-tin-oxide (ITO), tantalum pentoxide (Ta₂O₅), ceric oxide (CeO₂),yttrium oxide (Y₂O₃), europium oxide (Eu₂O₃), iron oxides such as(II)diiron(III) oxide (Fe₃O₄) and ferric oxide (Fe₂O₃), hafnium nitride(HfN), hafnium carbide (HfC), hafnium oxide (HfO₂), lanthanum oxide(La₂O₃), magnesium oxide (MgO), neodymium oxide (Nd₂O₃), praseodymiumoxide (Pr₆O₁₁), samarium oxide (Sm₂O₃), antimony trioxide (Sb₂O₃),silicon carbide (SiC), silicon nitride (Si₃N₄), silicon monoxide (SiO),selenium trioxide (Se₂O₃), tin oxide (SnO₂), tungsten trioxide (WO₃),combinations thereof, and the like.

Suitable low refractive index materials for dielectric layer 20 includesilicon dioxide (SiO₂), aluminum oxide (Al₂O₃), metal fluorides such asmagnesium fluoride (MgF₂), aluminum fluoride (AlF₃), cerium fluoride(CeF₃), lanthanum fluoride (LaF₃), sodium aluminum fluorides (e.g.,Na₃AlF₆ or Na₅Al₃F₁₄), neodymium fluoride (NdF₃), samarium fluoride(SmF₃), barium fluoride (BaF₂), calcium fluoride (CaF₂), lithiumfluoride (LiF), combinations thereof, or any other low index materialhaving an index of refraction of about 1.65 or less. For example,organic monomers and polymers can be utilized as low index materials,including dienes or alkenes such as acrylates (e.g., methacrylate),perfluoroalkenes, polytetrafluoroethylene (Teflon), fluorinated ethylenepropylene (FEP), combinations thereof, and the like.

The reflector layer 22 is formed on dielectric layer 20 by aconventional deposition process such as PVD, sputtering, or the like.The reflector layer 22 is formed to have a suitable thickness of about300-1000 Angstroms, and preferably a thickness of about 500-1000Angstroms. The reflector layer 22 is preferably composed of an opaque,highly reflective metal such as aluminum, silver, copper, gold,platinum, niobium, tin, combinations and alloys thereof and the like,depending on the color effects desired. It should be appreciated thatsemi-opaque metals such as grey metals become opaque at approximately350-400 Angstroms. Thus, metals such as chromium, nickel, titanium,vanadium, cobalt, and palladium, or cobalt-nickel alloys (which would bemagnetic), could also be used at an appropriate thickness for reflectorlayer 22.

In addition, reflector layer 22 can be composed of a magnetic materialsuch as a cobalt-nickel alloy, or can be formed of a semitransparentmaterial, to provide for machine readability for security verification.For example, machine readable information may be placed on a backingunderlying the optical coating, such as personal identification numbers(PINS), account information, business identification of source, warrantyinformation, or the like. In an alternative embodiment, reflector layer22 can be segmented to allow for partial viewing of underlyinginformation either visually or through the use of various optical,electronic, magnetic, or other detector devices. This allows fordetection of information below optical coating 16, except in thoselocations where reflector segments are located, thereby enhancing thedifficulty in producing counterfeits. Additionally, since the reflectorlayer is segmented in a controlled manner, the specific informationprevented from being read is controlled, providing enhanced protectionfrom forgery or alteration.

By using an absorber/dielectric/reflector design for color shiftingoptical coating 16, such as shown in FIG. 1A, high chroma variable coloreffects are achieved that are noticeable to the human eye. Thus, anobject having security article 10 applied thereto will change colordepending upon variations in the viewing angle or the angle of theobject relative to the viewing eye. As a result, the variation in colorswith viewing angle increases the difficulty to forge or counterfeitsecurity article 10. By way of example, the color-shifts that can beachieved utilizing color shifting optical coating 16 in accordance withthe present invention include, but are not limited to, gold-to-green,green-to-magenta, blue-to-red, green-to-silver, magenta-to-silver,magenta-to-gold, etc.

The color shifting properties of optical coating 16 can be controlledthrough proper design of the layers thereof. Desired effects can beachieved through the variation of parameters such as thickness of thelayers and the index of refraction of each layer. The changes inperceived color which occur for different viewing angles or angles ofincident light are a result of a combination of selective absorption ofthe materials comprising the layers and wavelength dependentinterference effects. The interference effects, which arise from thesuperposition of the light waves that have undergone multiplereflections and transmissions within the multilayered structure, areresponsible for the shifts in perceived color with different angles.

FIG. 1B depicts a security article 30 according to an alternativeembodiment of the present invention. The security article 30 includeselements similar to those discussed above with respect to securityarticle 10, including a light transmissive substrate 14 formed with anoptical interference pattern on a surface thereof, and a color shiftingoptical coating 16 that is a multilayer film. The optical coating 16 isformed, however, on the same side as the interference pattern onsubstrate 14 by conventional deposition processes. The optical coating16 includes an absorber layer 18 on the interference pattern, adielectric layer 20 on absorber layer 18, and a reflector layer 22 ondielectric layer 20. As shown in FIG. 1B, each of these layers formed onsubstrate 14 conforms to the shape of the interference pattern such as aholographic image.

FIG. 2A depicts a security article 40 according to another embodiment ofthe present invention. The security article 40 includes elements similarto those discussed above with respect to security article 10, includinga light transmissive substrate 14 formed with an optical interferencepattern 15 on an outer first surface thereof, and a color shiftingoptical coating 16 formed on an opposing second surface of substrate 14.The optical coating 16 is a multilayer film that includes an absorberlayer 18 and a dielectric layer 20 thereon, but does not include thereflector layer. This allows optical coating 16 to be transparent tolight incident upon the surface thereof, thereby providing for visualverification or machine readability of information below optical coating16 on a carrier substrate (not shown).

FIG. 2B depicts a security article 50 according to an alternativeembodiment of the present invention. The security article 50 includeselements similar to those discussed above with respect to securityarticle 40, including a light transmissive substrate 14 formed with anoptical interference pattern on a surface thereof, and a color shiftingoptical coating 16 that is a multilayer film. The optical coating 16 isformed, however, on the same side as the interference pattern onsubstrate 14 by conventional deposition processes. The optical coating16 includes an absorber layer 18 on the interference pattern, and adielectric layer 20 on absorber layer 18. This allows optical coating 16to be transparent to light incident upon the surface thereof, providingfor visual verification or machine readability of information on acarrier substrate.

FIG. 3 depicts a security article 60 according to a further embodimentof the present invention. The security article 60 includes elementssimilar to those discussed above with respect to security article 10,including a light transmissive substrate 14 formed with an opticalinterference pattern 15 on an outer first surface thereof, and a colorshifting optical coating 26 applied to an opposing second surface ofsubstrate 14. The color shifting optical coating 26 is formed from alayer of color shifting ink or paint that includes a polymeric mediuminterspersed with a plurality of optical interference flakes havingcolor shifting properties.

The color shifting flakes of optical coating 26 are formed from amultilayer thin film structure that includes the same basic layers asdescribed above for the optical coating 16 of security article 10. Theseinclude an absorber layer, a dielectric layer, and optionally areflector layer, all of which can be composed of the same materialsdiscussed above in relation to the layers of optical coating 16. Theflakes can be formed to have a symmetrical multilayer thin filmstructure, such as absorber/dielectric/reflector/dielectric/absorber, orabsorber/dielectric/absorber. Alternatively, the flakes can have anonsymmetrical structure, such as absorber/dielectric/reflector. Theflakes are formed so that a dimension on any surface thereof ranges fromabout 2 to about 200 microns.

Typically, the multilayer thin film structure is formed on a flexibleweb material with a release layer thereon. The various layers aredeposited on the web by methods well known in the art of forming thincoating structures, such as PVD, sputtering, or the like. The multilayerthin film structure is then removed from the web material as thin filmflakes, which can be added to a polymeric medium such as various pigmentvehicles for use as an ink or paint. In addition to the flakes,additives can be added to the inks or paints to obtain desired colorshifting results. These additives include lamellar pigments such asaluminum flakes, graphite, mica flakes, and the like, as well asnon-lamellar pigments such as aluminum powder, carbon black, and othercolorants such as organic and inorganic pigments, and colored dyes.

Suitable embodiments of the flake structure are disclosed in a copendingapplication Ser. No. 09/198,733, filed on Nov. 24, 1998, now U.S. Pat.No. 6,157,489 and entitled “Color Shifting Thin Film Pigments,” which isincorporated herein by reference. Other suitable embodiments of colorshifting or optically variable flakes which can be used in paints orinks for application in the present invention are described in U.S. Pat.Nos. 5,135,812, 5,171,363, 5,278,590, 5,084,351, and 4,838,648, thedisclosures of which are incorporated herein by reference.

For example, U.S. Pat. No. 5,135,812 discloses a symmetrical opticalmultilayer film which is composed either of transparent all-dielectricstacks, or transparent dielectric and semi-transparent metallic layeredstacks. In the case of an all-dielectric stack, the optical coating ismade of alternating layers of high and low index of refractionmaterials. In U.S. Pat. No. 5,278,590 to Phillips et al., a symmetricalthree-layer optical interference coating which can be formed into flakesis disclosed and includes first and second partially transmittingabsorber layers that have essentially the same composition andthickness, with a dielectric spacer layer located between the first andsecond absorber layers. The dielectric layer is composed of a materialhaving a low index of refraction such as magnesium fluoride.

The color shifting ink or paint utilized to form optical coating 26 onsecurity device 60 can be applied by conventional coating devices andmethods known to those skilled in the art. These include, for example,various printing methods such as silk screen, intaglio, gravure orflexographic methods, and the like. Alternatively, optical coating 26can be formed on security device 60 by coextruding a polymeric materialcontaining color shifting flakes, with the plastic material used to formsubstrate 14 having interference pattern 15.

FIG. 4 depicts a security article 70 according to another embodiment ofthe present invention. The security article 70 includes a lighttransmissive substrate 14 formed with an optical interference pattern 15on an outer surface thereof. A color shifting pigment is dispersedwithin substrate 14 and comprises a plurality of multilayer opticalinterference flakes, such as those described above with respect tosecurity article 40. The flakes are dispersed within the material thatforms substrate 14 prior to formation thereof. Preferably, the flakesare oriented so that they lie parallel to the planar back surface ofsubstrate 14 opposite from the outer surface thereof in order to providemaximum color shifting effects.

The various security articles as described above can be used in avariety of applications to provide for enhanced security measures suchas anticounterfeiting. The security articles can be utilized in the formof a label, tag, ribbon, security thread, tape, and the like, forapplication in a variety of objects such as security documents, monetarycurrency, credit cards, merchandise packaging, license cards, negotiablenotes, bank bonds, paper, plastic, or glass products, or other similarobjects.

The security articles of the invention can be transferred and attachedto various objects by a variety of conventional processes. For example,the security articles can applied to an object by use of a releaselayer. FIG. 5 shows security article 10 with a release layer 62 formedon substrate 14. The release layer 62 is of a suitable type to allowsecurity article 10 to be removed therefrom during the applicationprocess, such as by a hot-stamping process. The release layer 62 may bea polymeric material such as polyvinyl chloride, polystyrene,chlorinated rubber, acrylonitrile-butadiene-styrene copolymer,nitrocellulose, methyl methacrylate, acrylic copolymers, fatty acids,waxes, gums, gels, and mixtures thereof. The release layer is coupled toa carrier structure 64, which can be part of various manufacturing beltsor other processing structures that assist in transferring securityarticle 10 to the final structural element.

As shown in FIG. 6, the release layer is removed when security article10 has been applied to an object such as by hot-stamping, and thesecurity article is coupled to a carrier substrate 66 by way of anadhesive layer 68. The carrier substrate 66 may take the form of thefinal structural object to which security article 10 is to be bonded,such as those objects discussed above. The materials forming carriersubstrate 66 can be selected from plastics, cellulose, composites,polyester films, PET sheets, mylar sheets, cellophane, polypropylene,paper, rag/cotton, combinations thereof, and the like. The material ofadhesive layer 68 can be selected from acrylic-based polymers, UVactivated adhesives, ethylene vinyl acetate, polyamides, and the like.

FIGS. 7-8 depict the method and final structure of affixing a securityarticle, such as security article 30, to a carrier substrate 66 througha hot-stamping process. FIG. 7 shows security article 30 with a releaselayer 62 formed on one side of a light transmissive substrate 24, suchas an acrylic coating with an interference pattern formed thereon. Thesubstrate 24 may be composed of other materials such as those discussedabove relative to substrate 14, including polystyrene,polyacrylonitrile, polyvinyl chloride, and the like. The release layer62 is formed on the side opposite from optical coating 16 on theinterference pattern, and is attached to a carrier structure 64. Therelease layer 62 allows security article 30, including substrate 24,absorber layer 18, dielectric layer 20, and reflector layer 22, to bereleased from carrier structure 64 during the hot-stamping process.

Generally, carrier structure 64 can be composed of various materialswith various thicknesses which are known by those skilled in the art.For example, when carrier structure 64 is formed of PET, the thicknesspreferably ranges from about 10 μm to about 75 □m. Other materials andthickness ranges are applicable in light of the teachings containedherein.

Furthermore, the thickness of light transmissive substrate 24, whentaking the form of an acrylic material, can range from about 3 μm toabout 20 μm with an embossed surface. Generally, substrate 24 shouldhave a lower melting point or glass transition temperature than theoptical coating, while being transparent.

Prior to hot-stamping, an adhesive layer 68 is formed on reflector layer22, with the adhesive layer having a thickness of about 2 μm to about 20μm. As shown in FIG. 8, the release layer and carrier structure areremoved when security article 30 has been applied to an object such as acarrier substrate 66 by hot-stamping, with security article 30 beingcoupled to carrier substrate 66 by way of adhesive layer 68. The bondingof adhesive layer 68 against carrier substrate 66 occurs as a heatedmetal stamp (not shown) comes into contact with carrier structure 64.The heated metal stamp simultaneously forces adhesive layer 68 againstcarrier substrate 66 while heating adhesive layer 68 to more effectivelybond to carrier substrate 66. Furthermore, the heated metal stampsoftens release layer 62 thereby aiding in releasing security article 30from carrier structure 64 which is subsequently discarded. Once securityarticle 30 has been attached to carrier substrate 66, the image producedby security article 30 is viewed from substrate 24 toward opticalcoating 16.

The following examples are given to illustrate the present invention,and are not intended to limit the scope of the invention.

EXAMPLE 1

Optical coatings composed of color shifting flakes in a polymericvehicle were formed by a drawdown process on light transmissivesubstrates composed of PET films containing a holographic image. Thedrawdown vehicle included two parts lacquer/catalyst and one part colorshifting flakes. The color shifting flakes utilized had color shiftingproperties of green-to-magenta, blue-to-red, and magenta-to-gold.

EXAMPLE 2

A color shifting optical coating having a three-layer design was formedon an embossed transparent film to produce a security article. Theoptical coating was formed on the flat surface of the transparent filmon the side opposite from the embossed surface. The optical coating wasformed by depositing an absorber layer composed of chromium on the flatsurface of the transparent film, depositing a dielectric layer composedof magnesium fluoride on the absorber layer, and depositing a reflectorlayer of aluminum on the dielectric layer.

Alternatively, the aluminum layer can be deposited so that it istransparent. This would allow printed information on an object to beread underneath the optical coating. Further, the reflector layer canalternatively be composed of a magnetic material. Such a magneticfeature in the color shifting component when added to the holographiccomponent would give three independent security features to the securityarticle.

The embossed film and optical coating forming the security article canbe rigidly affixed to a carrier substrate, or can be attached to arelease layer so that the security article can be hot stamped to asurface of an object. In addition, the hot stamped image of the colorshifting thin film can be in the form of a pattern, as for example,dots, lines, logos, or other images. This pattern of optically variableeffects will add an even greater degree of deterrence to counterfeiting.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the forgoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A method of forming a security article, comprising the steps of:providing a light transmissive substrate having a first surface and anopposing second surface, the first surface having an opticalinterference pattern; and forming a color shifting optical coating onthe second surface of the substrate, the second surface beingsubstantially planar, wherein the color shifting coating provides anobservable discrete color shift such that the article has a firstbackground color at a first angle of incident light or viewing and asecond background color different from the first background color at asecond angle of incident light or viewing, the article exhibiting anoptical diffraction grating pattern effect or a holographic imagepattern effect in addition to the first and second background colors,and wherein a thickness of the substrate is in a range of 3 μm to 100μm.
 2. The method of claim 1, wherein the color shifting optical coatingis formed by depositing an absorber layer on the second surface of thesubstrate, depositing a dielectric layer overlying the absorber layerand depositing a reflector layer overlying the dielectric layer.
 3. Themethod of claim 1, wherein the color shifting optical coating is formedby depositing a first absorber layer on the second surface of thesubstrate, depositing a dielectric layer overlying the absorber layerand depositing a second absorber layer overlying the dielectric layer.4. The method of claim 1, wherein the color shifting optical coating isformed by applying a color shifting ink comprising a plurality ofmultilayer color shifting flakes dispersed in a polymeric medium to thesecond surface of the substrate.
 5. The method of claim 1, wherein thecolor shifting optical coating is formed on the second surface of thesubstrate by coextruding a color shifting material comprising aplurality of multilayer optical interference flakes dispersed in apolymeric medium, with a material forming the substrate.
 6. The methodof claim 1, further comprising the steps of forming a release layer onthe substrate, and of hot stamping the security article to an object.